View Single Post
Old 05-18-2011, 01:44 PM   #31 (permalink)
abogart's Avatar
Join Date: Mar 2011
Location: Michigan, USA
Posts: 50

EcoCorsica - '96 Chevrolet Corsica Base
90 day: 32.01 mpg (US)
Thanks: 13
Thanked 2 Times in 1 Post
Originally Posted by cleanspeed1 View Post
Old Mechanic brought it up first, here's an article on the Mazda Sky Activ engine (s).

Mazda SKYACTIV-G 1.3 Engine Details | Motorward

The gasser is direct injected.
Yes, it's direct injected. But it is still spark-ignited, and therefore subject to the limits of how lean a mixture can actually be ignited by a spark. Although, if it is truly the lack of a homogeneous mixture - such as Old_Mechanic mentioned - which is the limiting factor, wouldn't spark-ignited engines running on gaseous fuels (CNG, propane, etc.) be able to run very lean mixtures without compromising ignition?

Also, I notice that they market the engine as having 14.0:1 compression. Is this geometric compression ratio derived from engine design, or actual maximum WOT compression of the intake charge? If the variable valve timing allows the intake valve to remain open during some of the compression stroke (not sure if this happens at WOT), the compression ratio would be lower due to reduction of the amount of cylinder area compressed. Sometimes I think that automakers are just developing extensive new technologies to compensate for the flaws in one type of engine, while basically ignoring that another type is just more efficient.

I'm not trying to trash the new, innovative engine; I'm simply being skeptical and objective.

Originally Posted by mort View Post
Hi abogart,

Excellent post, but a clarification:

I don't know if you mean 75% of full throttle at high rpm or wot at 75% of red line or something else. It's very likely that the engine is most efficient near wot and very near the rpm for peak torque.

Sorry about that Mort!

Allow me to clarify... My definition of "load" was the manifold pressure (observed MAP as in. Hg on Scangauge) relative to atmospheric pressure (standard 29.9 in Hg). Regardless of throttle position, this most closely represents the percentage of maximum engine power being used at a given RPM. I rarely use more than 35% throttle (as observed TPS on SG) at any time. Under certain conditions (1400 RPM, TC locked), 29" MAP (100% load) can be obtained with as little at 25% throttle in my car.

Through actual experimentation, I found that accelerating at 22" MAP from 2000 to 2500 RPM gives less of a drop in average MPG than accelerating at any higher throttle setting in the same RPM range. I initially derived the 75% load value by figuring 22 / 29. However, I just realized that this would be inaccurate because the MAP does not go all the way to 0 at closed throttle, but closer to 7", depending on RPM. So I guess that I could reduce the scale to 22 and figure 15 / 22 which would be about 68% load... But I think there is some math involved here which is way beyond my current level of understanding .

Anyway, I found 22" to be the "sweet spot" which yields the best MPG for the way that I drive. Note that the car does have an automatic transmission (doh!) and these numbers may be construed by torque converter losses or any number of other factors. Not to mention that I really cannot do any RPM vs. throttle experimentation outside what the PCM will allow . You might very well be right though Mort, the auto trans. really complicates things.

  Reply With Quote